Modern power electronics systems, that require conversion of AC power from an electricity utility company AC supply to DC power for use by a process, frequently use a Switched Mode Power Supply (SMPS) for the required inversion.
A typical known SMPS 10 as illustrated in FIG. 1 comprises an inverter 11 which has input ports electrically connected by first leads 120 to an AC energy source 12 to generate a higher frequency AC inverter drive waveform 112, shown in a first inset 111 of FIG. 1, which is fed from an output of the inverter 11 via second leads 110 towards a 1:N XFMR transformer rectifier unit 13. A secondary winding 133 of the transformer is connected by leads 130 to Dn diodes 14 to provide a desired DC voltage trace 162, shown in a second inset 161 of FIG. 1, across a load 16. For reasons explained below, an L1 common mode inductor 18 is connected in series between the inverter 11 and the transformer rectifier unit 13, so that the output leads 110 from the inverter are connected to an input of the L1 common mode inductor 18 and output leads 150 from the inductor 18 electrically connect the inductor 18 to a primary winding 131 of the transformer 13.
As shown in FIG. 1, a first stray capacitance Cs1 exists between earth and the first leads 120 connecting the energy source 12 to the inverter 11. A second stray capacitance Cs2a exists between the primary winding 131 and a core or former 132 of the transformer rectifier unit 13. A third stray capacitance Cs3a exists between the primary winding 131 and the secondary winding 133 of the transformer rectifier unit 13 and a fourth stray capacitance Cs4a exists between the core or former 132 and the secondary winding 133 of the transformer rectifier unit 13.
The Dn diodes 14 are connected across the output terminals of the secondary winding, typically, but not restricted to, a bridge rectifier arrangement, 133 and an Rmon monitor resistance 15 is connected in series between earth and a first terminal of the Dn diodes 14 connected to a first output terminal of the secondary winding 133, to monitor load current pulses, a corresponding voltage trace 152 across the monitor resistance 15 being illustrated in a third inset 151 of FIG. 1. The load 16 is connectable between the second output terminal of the Dn diodes 14 and earth.
With high power systems, for example with a DC output power above 30 kW, the SMPS 10 and the XFMR transformer rectifier unit 13 with diodes 14 can be physically large, for example greater than 250 liters, so that stray capacitances Cs1 and Cs2a to Cs4a of the various component parts are significant, for example greater than 10 nF.
Separation between such large volume items is also typically large, for example 3 meters or more, and with this length of electrical connectors 110, 120, 130, 150 therebetween, stray inductance of, for example, 2 or 3 μH is introduced.
The inverter drive waveforms 112 are pulsed and their rise and fall times Tr and Tf, can be relatively rapid, for example of the order of 1 μs or less. The inverter output peak voltages ±Epk, shown in the first inset 111, typically have values up to 1 kV.
Stray currents I are related to the stray capacitance C and rise and fall rates dV/dt by the formula:I=C*dV/dt, so for 10 nF stray capacitance and a 1 μs rise time from zero to 1 kV, the peak resultant stray current is of the order of 10 A.
These stray currents flow through the connector leads 110, 120, 130, 150 which, as indicated above, are typically several meters long, presenting a very high risk of EMC problems, both internal to subsystems of the inverter and externally to co-located or adjacent equipment.
As shown in FIG. 1, in an attempt to reduce the impact of this stray current, an L1 common mode choke 18 is often used between the inverter 11 and the transformer rectifier unit 13 to reduce an amplitude of first stray current pulses, Is1a, that flow in a loop from earth through the first stray capacitance Cs1, the inverter 11 and via the second and third stray capacitances Cs2a, Cs3a, the XFMR windings 132, 133 and the Rmon monitor resistor 15 back to earth.
In applications where inverters are used to drive electric motors, these currents flowing in stray capacitances have been known to be sufficient to damage insulation and bearings of the motor. Moreover, unlike a motor, the diodes Dn of an XFMR provide a potential source of a second stray current Is2a. 
There are many well-known arrangements of rectifiers for converting AC signals to DC signals or uni-directional pulses. In all cases, as the current in the Dn rectifier diodes 14 falls to zero and the voltage across them reverses, a “recovery current”, as the diode re-establishes reverse voltage blocking, can produce very rapid transients. These reverse currents form a second stray current Is2a that flows through the fourth stray capacitance Cs4, the secondary winding 133 of XFMR transformer 13 and the Rmon monitor resistance 15 to add to the problem of undesirable noise voltages.
The net result, at the very least, is that the voltage across Rmon monitor resistance 15 is distorted with significant transient voltages Vb and −Vc on the leading and trailing edges of the pulses as shown in the third inset 151 of FIG. 1. This can disturb current monitoring signals that may be required to be monitored precisely for process control, and, in addition, the voltages developed across the stray capacitances Cs1 and Cs2a to Cs4a of residual elements may put excessive stress on dielectric materials used in the construction of XFMR transformer 13.
Furthermore, the stray capacitances Cs2a to Cs4a are not simple capacitances in that they represent the capacitance of transformer windings to surrounding structures and as such will have significant inductance in series with the capacitance. This further complicates the situation and it is frequently found that transient voltages Vb and Vc in the Rmon monitoring resistance 15 manifest themselves as large amplitude, high frequency, typically 0.2 to 5 MHz, damped oscillations that can persist for a large portion of a pulse duration.
The use of an electrostatic screen in a transformer is known: for example, in a transformer described in the Applicant's international patent application WO 2010/013049, such an electrostatic screen is used. However, this is not completely effective in the present application because there are two predominant paths as described above in which the currents Is1a and Is2a in stray capacitances Cs1 and Cs2a to Cs4a can flow.
It is desirable to reduce effects of these stray currents.